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Abstract:

Embodiments of an apparatus comprising a pixel array comprising a
plurality of macropixels. Each macropixel includes a pair of first pixels
each including a color filter for a first color, the first color being
one to which pixels are most sensitive, a second pixel including a color
filter for a second color, the second color being one to which the pixels
are least sensitive and a third pixel including a color filter for a
third color, the third color being one to which pixels have a sensitivity
between the least sensitive and the most sensitive, wherein the first
pixels each occupy a greater proportion of the light-collection area of
the macropixel than either the second pixel or the third pixel.
Corresponding process and system embodiments are disclosed and claimed.

Claims:

1. An apparatus comprising: a pixel array comprising a plurality of
macropixels formed on a substrate, each macropixel including: a pair of
first pixels each including a color filter for a first color, the first
color being one to which pixels are most sensitive, a second pixel
including a color filter for a second color, the second color being one
to which the pixels are least sensitive, and a third pixel including a
color filter for a third color, the third color being one to which pixels
have a sensitivity between the least sensitive and the most sensitive,
wherein the first pixels each occupy a greater proportion of the
light-collection area of the macropixel than either the second pixel or
the third pixel.

2. The apparatus of claim 1 wherein the first pixels each occupy between
about 20 percent and about 40 percent of the light-collection area of the
macropixel, the second pixel occupies between about 10 percent and about
20 percent of the light-collection area of the macropixel, and the third
pixel occupies between about 10 percent and about 20 percent of the
light-collection area of the macropixel.

3. The apparatus of claim 1 wherein the first pixels each occupy about 30
percent of the light-collection area of the macropixel, the second pixel
occupies about 20 percent of the light-collection area of the macropixel,
and the third pixel occupies about 20 percent of the light-collection
area of the macropixel.

4. The apparatus of claim 1 wherein the first color is green, the second
color is blue and the third color is red.

5. The apparatus of claim 1 wherein the first color is yellow, the second
color is magenta and the third color is cyan.

6. The apparatus of claim 1 wherein the pair of first pixels have a shape
that is octagonal.

7. The apparatus of claim 1 wherein the macropixel is quadrilateral and
includes four pixels, with the first pixels in opposite corners and the
second pixel in the opposite corner from the third pixel.

8. The apparatus of claim 1 wherein the first, second and third pixels
are formed on the front side of the substrate and the filters for the
first, second and third color are formed on the front side of the
substrate.

9. The apparatus of claim 1 wherein the first, second and third pixels
are formed on the front side of the substrate and the first, second and
third color filters are formed on the backside of the substrate.

10. The apparatus of claim 1, further comprising control circuitry or
readout circuitry coupled to the pixels of the pixel array.

11. The apparatus of claim 10 wherein the control circuitry can generate
one or more of a transfer (TX) signal, a reset (RST) signal and a select
signal for each pixel within the pixel array.

12. A system comprising: an image sensor formed in a substrate, wherein
the image sensor has a pixel array including a plurality of macropixels,
each macropixel comprising: a pair of first pixels each including a color
filter for a first color, the first color being one to which pixels are
most sensitive, a second pixel including a color filter for a second
color, the second color being one to which the pixels are least
sensitive, and a third pixel having a filter for a third color, the third
color being one to which pixels have a sensitivity between the least
sensitive and the most sensitive, wherein the first pixels each occupy a
greater proportion of the light-collection area of the macropixel than
the second pixel or the third pixel; and control circuitry and processing
circuitry coupled to the image sensor to read out and process a signal
from the pixel array.

13. The system of claim 12 wherein the first pixels each occupy between
about 20 percent and about 40 percent of the light-collection area of the
macropixel, the second pixel occupies between about 10 percent and about
30 percent of the light-collection area of the macropixel, and the third
pixel occupies between about 10 percent and about 30 percent of the
light-collection area of the macropixel.

14. The system of claim 12 wherein the first pixels each occupy about 30
percent of the light-collection area of the macropixel, the second pixel
occupies about 20 percent of the light-collection area of the macropixel,
and the third pixel occupies about 20 percent of the light-collection
area of the macropixel.

15. The system of claim 12 wherein the first color is green, the second
color is blue and the third color is red.

16. The system of claim 12 wherein the first color is yellow, the second
color is magenta and the third color is cyan.

17. The system of claim 12 wherein the pair of first pixels have a shape
that is octagonal.

18. The system of claim 12 wherein the first, second and third pixels are
formed on the front side of the pixel array and the filters for the
first, second and third color are formed on the front side of the pixel
array.

19. The system of claim 12 wherein the first, second and third pixels are
formed on the front side of the pixel array and the first, second and
third color filters are formed on the backside of the pixel array.

20. The system of claim 12, further comprising an analog-to-digital
converter (ADC) coupled to the processing circuitry.

21. The system of claim 20 wherein the processing circuitry includes a
digital signal processor coupled to the ADC.

22. The system of claim 12 wherein each pixel further comprises: a
photodetector formed in the substrate; a floating diffusion formed in the
substrate; and a transfer gate formed on the substrate between a
photodetector and the floating diffusion.

Description:

TECHNICAL FIELD

[0001] This disclosure relates generally to image sensors and in
particular, but not exclusively, relates to an improved photodiode area
allocation for CMOS image sensors.

BACKGROUND

[0002] Image sensors have become ubiquitous. They are widely used in
digital still cameras, cellular phones and security cameras, as well as
medical, automobile, and other applications. The technology used to
manufacture image sensors, and in particular complementary
metal-oxide-semiconductor ("CMOS") image sensors ("CIS"), has continued
to advance at great pace. For example, the demands of higher resolution
and lower power consumption have encouraged the further miniaturization
and integration of these image sensors. Miniaturization also contributes
to cost reduction of image sensors.

[0003] One field of application in which size and image quality is
particularly important is medical applications (e.g., endoscopes). For
medical applications the image sensor chip must typically be small while
providing a high quality image. In order to achieve these
characteristics, for a given chip size the photosensitive apertures
should be as large as possible while peripheral circuitry should be as
limited as possible.

[0004] The pixel (picture element) fill factor denotes that fraction of
the area of a pixel that is sensitive to light. Pixel pitch is the
physical distance between the pixels in an imaging device. Pixel fill
factor has become smaller as pixel pitch has been reduced because the
active circuit elements and metal interconnects consume increasing
amounts of area in each pixel. One way to address the loss of fill factor
is to use a microscale lens (microlens) directly above each pixel to
focus the light directly towards the photosensitive portion of the area
within the pixel. Another way to address the loss of fill factor is to
use backside-illuminated ("BSI") image sensors which place the active
pixel circuit elements and metal interconnects on a frontside of an image
sensor die, and a photosensitive element within the substrate facing a
backside of an image sensor die. For BSI image sensors, the majority of
photon absorption occurs near the backside silicon surface. However, a
solution that provides larger individual pixel area on the same silicon
area would improve BSI image sensors as well as frontside illuminated
image sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Non-limiting and non-exhaustive embodiments of the present
invention are described with reference to the following figures, wherein
like reference numerals refer to like parts throughout the various views
unless otherwise specified.

[0007]FIG. 2 is a circuit diagram of pixel circuitry of two 4T pixels
within a conventional CMOS image sensor.

[0008]FIG. 3 is a schematic representation of a Bayer-patterned pixel
array showing a grouping of pixels into a macropixel block.

[0009]FIG. 4A is schematic representation of an embodiment of a pixel
array including a Bayer-patterned macropixel block.

[0010]FIG. 4B is a schematic representation of a pixel array including an
alternative embodiment of a Bayer-patterned macropixel block.

[0011]FIG. 4c is a schematic representation of a pixel array including
another alternative embodiment of a macropixel block.

[0012]FIG. 5A is cross-sectional view of a pair of frontside-illuminated
pixels.

[0013]FIG. 5B is a cross-sectional view of a pair of backside-illuminated
pixels.

[0014]FIG. 6 is a block diagram of an embodiment of an imaging system
including a pixel array having macropixel blocks according to the present
invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0015] Embodiments of an apparatus, process and system for an image sensor
with improved photodiode area allocation are described herein. In the
following description numerous specific details are set forth to provide
a thorough understanding of the embodiments. One skilled in the relevant
art will recognize, however, that the techniques described herein can be
practiced without one or more of the specific details, or with other
methods, components, materials, etc. In other instances, well-known
structures, materials, or operations are not shown or described in detail
to avoid obscuring certain aspects.

[0016] Reference throughout this specification to "one embodiment" or "an
embodiment" means that a particular feature, structure, or characteristic
described in connection with the embodiment is included in at least one
embodiment of the present invention. Thus, the appearances of the phrases
"in one embodiment" or "in an embodiment" in various places throughout
this specification are not necessarily all referring to the same
embodiment. Furthermore, the particular features, structures, or
characteristics may be combined in any suitable manner in one or more
embodiments.

[0017]FIG. 1 illustrates an embodiment of a CMOS image sensor 100
including a color pixel array 105, readout circuitry 110, function logic
115, and control circuitry 120. Color pixel array 105 is a
two-dimensional ("2D") array of imaging sensors or pixels (e.g., pixels
P1, P2 . . . , Pn) having X number of pixel columns and Y number of pixel
rows. In one embodiment, each pixel is a complementary
metal-oxide-semiconductor ("CMOS") imaging pixel. Color pixel array 105
may be implemented as either a frontside-illuminated pixel array or a
backside-illuminated image pixel array. As illustrated, each pixel is
arranged into a row (e.g., rows R1 to Ry) and a column (e.g., column C1
to Cx) to acquire image data of a person, place, or object, which can
then be used to render a 2D image of the person, place, or object. After
each pixel has acquired its image data or image charge, the image data is
read out by readout circuitry 110 and transferred to function logic 115.
Readout circuitry 110 may include amplification circuitry,
analog-to-digital ("ADC") conversion circuitry, or otherwise. Function
logic 115 may simply store the image data or even manipulate the image
data by applying post-image effects (e.g., crop, rotate, remove red eye,
adjust brightness, adjust contrast, or otherwise). Control circuitry 120
is coupled to pixel array 105 to control operational characteristic of
color pixel array 105. For example, control circuitry 120 may generate a
shutter signal for controlling image acquisition.

[0018] Color pixel array 105 assigns color to each pixel through the use
of a color filter array ("CFA"). CFAs assign a separate primary color to
each pixel by placing a filter of that color over the pixel. As photons
pass through a filter of a certain primary color to reach the pixel, only
wavelengths of that primary color will pass through. All other
wavelengths will be absorbed. Primary colors are a set of colors
identified by science as being the building blocks for all other colors.
Examples of primary colors include red, green and blue (commonly referred
to as RGB) and cyan, magenta and yellow (commonly referred to as CMY). In
the RGB color model, for example, combining varying amounts of red, green
and blue will create all the other colors in the spectrum.

[0019] Numerous types of CFAs have been developed for different
applications. CFA patterns are almost exclusively comprised of identical
square pixel elements, referred to as micropixels, arranged in
rectangular X, Y patterns. Hexagonal and octagonal pixels have been
proposed, but repeating pixel units, referred to as macropixels, are
usually found in groups of four. In the vast majority of digital camera
image sensors, the most popular CFA is the Bayer pattern. Using a
checkerboard pattern with alternating rows of filters, the Bayer pattern
has twice as many green pixels as red or blue pixels, and they are
arranged in alternating rows of red wedged between greens, and of blue
wedged between greens. This pattern takes advantage of the human eye's
predilection to see green luminance as the strongest influence in
defining sharpness. What's more, the Bayer pattern produces identical
images regardless of how you hold the camera--in landscape or portrait
mode.

[0020]FIG. 2 is a circuit diagram illustrating an embodiment of the pixel
circuitry 200 of two four-transistor ("4T") pixels within a pixel array
105. Pixel circuitry 200 represents one possible architecture for
implementing each pixel within color pixel array 105 of FIG. 1, but
embodiments of the present invention are not limited to 4T pixel
architectures; rather, one of ordinary skill in the art having the
benefit of the instant disclosure will understand that the present
teachings are also applicable to 3T designs, 5T designs, and various
other pixel architectures. In FIG. 2, pixels Pa and Pb are arranged in
two rows and one column. The illustrated embodiment of each pixel
circuitry 200 includes a photosensitive element PD, a transfer transistor
T1, a reset transistor T2, a source-follower ("SF") transistor T3 and a
select transistor T4. During operation of each pixel, transfer transistor
T1 receives a transfer signal TX, which transfers the charge accumulated
in photosensitive element PD to a floating diffusion node FD. Reset
transistor T2 is coupled between a power rail VDD and the floating
diffusion node FD to reset the FD (e.g., discharge or charge the FD to a
preset voltage) under control of a reset signal RST. The floating
diffusion node FD is coupled to control the gate of SF transistor T3. SF
transistor T3 is coupled between the power rail VDD and select transistor
T4. SF transistor T3 operates as a source-follower providing a high
impedance output from the pixel. Finally, select transistor T4
selectively couples the output of pixel circuitry 200 to the readout
column line under control of a select signal SEL. In one embodiment of a
pixel array 105, the TX signal, the RST signal, and the SEL signal are
generated by control circuitry 120.

[0021]FIG. 3 shows a portion 300 of pixel array 105 and a Bayer-patterned
macropixel 310 including four pixels (hereafter referred to as
"micropixels" to distinguish them from the macropixel). The
Bayer-patterned macropixels are positioned with the pixel array with a
uniform X and Y separation distance Ip. In the illustrated
embodiment, each micropixel within macropixel 310 occupies substantially
the same light collection area--that is, each micropixel occupies
substantially 25% of the light-collection area of the macropixel. A
Bayer-patterned macropixel is a repeating unit of a color filter array
(CFA) for arranging red (R), green (G) and blue (B) color filters over an
array of photosensitive elements. When a Bayer-patterned pixel array's
charge is read out, the colors are recorded sequentially line by line.
One line would be BGBGBG, followed by a line of GRGRGR, and so forth.
This is known as sequential RGB.

[0022] An important aspect of any imaging system is the signal-to-noise
ratio (SNR). Signal-to-noise ratio may become smaller as pixels are
miniaturized since the noise level may remain the same while the signal
becomes smaller as the collection area becomes smaller. Gain can be used
to boost the output signal, but any such gain element would increase the
noise along with the signal because the amplification circuit itself also
produces noise (e.g., Johnson noise produced by the components). In the
prior art, attempts have been made to improve color image quality by
improving the SNR of colored pixels, and especially those colors which
conventionally produce less signal. Pixels in RGB-type color image
sensors typically have varying responses to different colors. For pixels
with substantially equal light-collection areas, the ratio between
signals from the red pixels (Vr), green pixels (Vg), and blue pixels (Vb)
is typically in the rough ratio of 2.5 Vb:1.5 Vr:1.0 Vg, meaning that the
blue pixels produce 2.5 times less signal than the green pixels and the
red pixels produce 1.5 times less signal than the green pixels. Thus, for
a given pixel area blue pixels produce the smallest signal, green pixels
produce the largest signal, and red pixels produce signals somewhere
between the green pixels and the blue pixels.

[0023] One conventional solution to improve the SNR of image sensors seeks
to equalize the output signal of all pixels in a macropixel by changing
the area of each micropixel to collect more or less photons to compensate
for the difference in signals produced by the different colors. Since it
is important to maintain or increase the image sensor resolution, the
micropixel pitch must be kept the same or decreased. In that case the
macropixel area allocation to the different colors in its makeup may be
changed from an equal allocation. U.S. Pat. No. 6,137,100, for example,
allocates the largest light-collection area to the blue color since it
produces the lowest signal level. The smallest light-collection area is
allocated to the green color which produces the highest signal level.

[0024]FIG. 4A shows a portion 400 of a pixel array including an
embodiment of a Bayer-patterned macropixel 410 having four micropixels.
The Bayer-patterned macropixels are positioned within a pixel array with
the same uniform X and Y separation distance Ip as shown in FIG. 3.
In contrast to macropixel 310, in which each micropixel is allocated
substantially 25% of the macropixel's light-collection area, macropixel
410 allocates different portions of the macropixel's light-collection
area to the different colors. The illustrated embodiment allocates about
30% of the macropixel's light-collection area to each of the two green
micropixels and about 20% each to the blue and red micropixels. Other
embodiments can allocate the macropixel's light-collection area
differently. For example, in other embodiments the green pixels can each
occupy between about 20% and about 40% of the light-collection area of
the macropixel, the blue pixel can occupy between about 10% and about 30%
of the light-collection area, and the red pixel can occupy between about
10% and about 30% of the light-collection area. In some embodiments both
green micropixels can have the substantially the same light-collection
area allocation and the blue and red micropixels can have substantially
equal but smaller allocations than the green micropixels, but in other
embodiments this need not be the case.

[0025] An object of embodiments of a pixel array that include a macropixel
such as macropixel 410 is to improve SNR by increasing the collection
area allocated to the green color at the expense of the collection area
allocated to the blue and red color. This arrangement takes advantage of
the human eye's predilection to see green luminance as the strongest
influence in defining sharpness. What's more, it produces identical
images regardless of how you hold the camera--in landscape or portrait
mode. This solution is particularly advantageous for applications
requiring high resolution and high SNR with minimum pixel size where
color accuracy is a lower priority. Such applications may be found in
automotive, security, or machine vision systems.

[0026]FIG. 4B illustrates a portion 425 of a pixel array including an
alternative embodiment of a macropixel 435. Macropixel 435 is in many
respects similar to macropixel 410. The principal difference between
macropixels 410 and 435 is in the shape of the green micropixels. In an
embodiment where the green micropixels are assigned a proportion of the
macropixel's light-collection area so large as to cause their corners to
overlap, their shape can be made octagonal as illustrated to maintain a
measure of isolation between the green pixels.

[0027]FIG. 4c illustrates a portion 450 of a pixel array with another
alternative embodiment of a macropixel 460. Macropixel 460 is in many
respects similar to macropixels 410 and 435. The principal difference
between macropixel 460 and macropixels 410 and 425 is in the primary
colors that are used: macropixels 410 and 435 use red, green and blue as
primary colors, while macropixel 460 instead uses magenta, yellow and
cyan. Magenta, yellow and cyan are a common alternative set of primary
colors that can be used to produce color images. As with the RGB color
set, for the same pixel area magenta, yellow and cyan produce different
signal levels: yellow produces the largest signal, magenta the smallest
signal, and cyan produces a signal in between yellow and magenta.
Analogously to macropixel 410, then, macropixel 460 allocates about 30%
of the macropixel's light-collection to each of the two yellow
micropixels and about 20% each to the magenta and cyan micropixels.

[0028] As with macropixels 410 and 435, other embodiments of macropixel
460 can allocate the macropixel's light-collection area differently. For
example, in other embodiments the yellow pixels can each occupy between
about 20% and about 40% of the light-collection area of the macropixel,
the magenta pixel can occupy between about 10% and about 30% of the
light-collection area, and the cyan pixel can occupy between about 10%
and about 30% of the light-collection area. In some embodiments both
yellow micropixels can have the substantially the same light-collection
area allocation and the magenta and cyan micropixels can have
substantially equal but smaller allocations than the yellow micropixels,
but in other embodiments this need not be the case. As in macropixel 435,
in an embodiment where the yellow micropixels in macropixel 460 are
assigned a proportion of the macropixel's light-collection area so large
as to cause their corners to overlap, their shape can be made octagonal
to maintain a measure of isolation between the yellow pixels.

[0029] FIGS. 5A-5B illustrate cross-sections of frontside-illuminated and
backside-illuminated embodiments of a pair of micropixels in a CMOS image
sensor. The macropixel embodiments previously described can be
implemented as a frontside-illuminated embodiment as shown in FIG. 5A or
a backside-illuminated embodiment as shown in FIG. 5B. FIG. 5A
illustrates an embodiment of pixels 500 in a frontside-illuminated CMOS
image sensor. The front side of pixels 500 is the side of substrate 502
upon which the pixel circuitry is disposed and over which metal stack 504
for redistributing signals is formed. Metal layers M1 and M2 are
patterned in such a manner as to create an optical passage through which
light incident on the frontside-illuminated pixels 500 can reach the
photosensitive or photodiode ("PD") region 506. To implement a color
image sensor, the front side includes color filters 508, each disposed
under a microlens 510 that aids in focusing the light onto PD region 506.
To implement the embodiments of macropixels 410, 435 or 460, each color
filter 508 has the appropriate color and each of pixels 500 is sized to
have the light-collection area allocation that corresponds to its color.

[0030]FIG. 5B illustrates an embodiment of pixels 550 in a
backside-illuminated CMOS image sensor. As with the pixels 500, the front
side of pixels 550 is the side of substrate 552 upon which the pixel
circuitry is disposed and over which metal stack 554 for redistributing
signals is formed. The backside is the side of substrate 552 opposite the
front side. To implement a color image sensor, the backside includes
color filters 558, each disposed between the backside and microlenses
560. Microlenses 560 aid in focusing the light onto PD region 556. By
illuminating the backside of pixels 550, the metal interconnect lines in
metal stack 554 would not obscure the path between the object being
imaged and the collecting areas, resulting in greater signal generation
by PD regions 556. To implement the embodiments of macropixels 410, 435
or 460, each color filter 558 has the appropriate color and each of
pixels 550 is sized to have the light-collection area allocation that
corresponds to its color.

[0031]FIG. 6 illustrates an embodiment of an imaging system 600. Optics
601, which can include refractive, diffractive or reflective optics or
combinations of these, are coupled to image sensor 602 to focus an image
onto the pixels in pixel array 604 of the image sensor. Pixel array 604
captures the image and the remainder of imaging system 600 processes the
pixel data from the image.

[0032] Image sensor 602 comprises a pixel array 604 and a signal reading
and processing circuit 610. The pixels in pixel array 604 can be
frontside-illuminated or backside-illuminated pixels as shown in FIGS.
5A-5B and can be grouped into the macropixel embodiments shown in FIGS.
4A-4C. During operation of pixel array 604 to capture an image, each
pixel in pixel array 604 captures incident light (i.e., photons) during a
certain exposure period and converts the collected photons into an
electrical charge. The electrical charge generated by each pixel can be
read out as an analog signal, and a characteristic of the analog signal
such as its charge, voltage or current will be representative of the
intensity of light that was incident on the pixel during the exposure
period.

[0033] In one embodiment pixel array 604 is two-dimensional and includes a
plurality of pixels arranged in rows 606 and columns 608. Illustrated
pixel array 604 is regularly shaped, but in other embodiments the array
can have a regular or irregular arrangement different than shown and can
include more or less pixels, rows, and columns than shown. Moreover, in
different embodiments pixel array 604 can be a color image sensor
including red, green, and blue pixels or can be a magenta-cyan-yellow
image sensor.

[0034] Image sensor 602 includes signal reading and processing circuit
610. Among other things, circuit 610 can include circuitry and logic that
methodically reads analog signals from each pixel, filters these signals,
corrects for defective pixels, and so forth. In an embodiment where
circuit 610 performs only some reading and processing functions, the
remainder of the functions can be performed by one or more other
components such as signal conditioner 612 or DSP 616. Although shown in
the drawing as an element separate from pixel array 604, in some
embodiments reading and processing circuit 610 can be integrated with
pixel array 604 on the same substrate or can comprise circuitry and logic
embedded within the pixel array. In other embodiments, however, reading
and processing circuit 610 can be an element external to pixel array 604
as shown in the drawing. In still other embodiments, reading and
processing circuit 610 can be an element not only external to pixel array
604, but also external to image sensor 602.

[0035] Signal conditioner 612 is coupled to image sensor 602 to receive
and condition analog signals from pixel array 604 and reading and
processing circuit 610. In different embodiments, signal conditioner 612
can include various components for conditioning analog signals. Examples
of components that can be found in the signal conditioner include
filters, amplifiers, offset circuits, automatic gain control, etc. In an
embodiment where signal conditioner 612 includes only some of these
elements and performs only some conditioning functions, the remaining
functions can be performed by one or more other components such as
circuit 610 or DSP 616. Analog-to-digital converter (ADC) 614 is coupled
to signal conditioner 612 to receive conditioned analog signals
corresponding to each pixel in pixel array 604 from signal conditioner
612 and convert these analog signals into digital values.

[0036] Digital signal processor (DSP) 616 is coupled to analog-to-digital
converter 614 to receive digitized pixel data from ADC 614 and process
the digital data to produce a final digital image. DSP 616 can include a
processor and an internal memory in which it can store and retrieve data.
After the image is processed by DSP 616, it can be output to one or both
of a storage unit 618 such as a flash memory or an optical or magnetic
storage unit and a display unit 620 such as an LCD screen.

[0037] The above description of illustrated embodiments of the invention,
including what is described in the abstract, is not intended to be
exhaustive or to limit the invention to the precise forms disclosed.
While specific embodiments of, and examples for, the invention are
described herein for illustrative purposes, various modifications are
possible within the scope of the invention, as those skilled in the
relevant art will recognize.

[0038] These modifications can be made to the invention in light of the
above detailed description. The terms used in the following claims should
not be construed to limit the invention to the specific embodiments
disclosed in the specification. Rather, the scope of the invention is to
be determined entirely by the following claims, which are to be construed
in accordance with established doctrines of claim interpretation.